This invention relates to display devices in general, and in particular to an apparatus and method for biasing a drive voltage in a display device having at least one cathode.
A problem in driving cathode ray tube display devices is that the black level, or cutoff voltage, of such devices varies significantly between two or more devices even in case where the displays are made according to identical manufacturing specifications. Moreover, the cutoff voltage of an individual display device is known to "drift" over the lifetime of a device as a result of aging, and during operation of the device as a result of temperature fluctuations, humidity and other factors.
Given any image presentation, the contrast of the image is defined by the equation: ##EQU1##
Where C is the contrast percentage
MAX is the maximum brightness in the image PA1 MIN is the minimum brightness in the image
The only way that contrast can reach 100% is if the numerator and denominator are equal, which can only happen if MIN is zero. In a display device, as long as MIN can approach zero, a full range of contrast can be generated, regardless of the value of the MAX. Thus, a minimal black level that does not drift extends the contrast of a display device.
Many cathode ray tube display devices have a mechanical adjustment component, normally a potentiometer, for biasing the device's drive voltage in accordance with the particular cutoff voltage of the device. Normally, the bias of the drive voltage is adjusted once when the device is first manufactured, and is generally not adjusted thereafter. Therefore, the responsiveness of such manually controlled devices to applied drive voltages changes over the lifetime of the device and during the course of operation with drifting cutoff voltage.
A method known as Automatic Kinescope Bias (AKB) has been developed to adaptively bias a display device in accordance with detected changes in cutoff voltage. In the technique of Automatic Kinescope Bias, two test voltages are successively applied to the cathode or grid of a cathode ray tube. The first test voltage is selected to result in a zero, black level beam current, while the second voltage in known relation to the first test voltage, is selected to result in a predetermined white level voltage. The measured cathode current resulting from application of the first voltage is compared to the measured cathode current resulting from application of the second voltage to determine an estimation of actual beam current A biasing voltage for biasing test and drive voltages is adjusted accordingly. AKB successfully corrects all fixed cathode leakage currents, including cathode circuit to grid circuit, further referred to herein as DC leakage.
While AKB techniques greatly improve the responsiveness and contrast control of cathode ray display tubes, they do not adequately characterize all components of leakage current which is influenced by the relative positioning of components of a display device whose positioning changes over time as the device encounters thermal and mechanical stresses. Because known AKB circuits do not take into account all components of current leakage, display tubes having only AKB biasing are commonly observed to erroneously generate a beam current with a drive voltage biased to generate a black screen, or conversely to erroneously generate a black screen when biased to generate a white level beam current.
For applications requiring precise control over electron beam intensity in a cathode ray tube, including for example ophthalmic diagnostic, video medical diagnostic, and telemedicine applications, there is an urgent need for a drive voltage biasing circuit which overcomes the limitations inherent in prior art biasing methods.